The present invention relates to a developing apparatus incorporated in an image forming device such as a copy machine, a printer, a facsimile, etc.
Regarding developing methods utilized for electrophotographic devices, it is well known that digital printers, digital copiers, etc. employ a reversal developing method, while conventional copiers, etc. employ a standard developing method. In the reversal developing method, after a latent electrostatic image is formed on an image bearing element (hereinafter, referred as a photoreceptor drum) through processes of electrostatic charging and exposure of light emitted from a light source such as lasers, LED (Light Emitting Diode), etc., a visible toner image is developed, in which toner is charged at the same polarity as the charged photoreceptor drum. For instance, when the photoreceptor drum is charged negatively, a visible toner image is formed on the photoreceptor drum by developing with toner also charged negatively in accordance with the pattern of electrostatic potential differences generated from the exposure of light. After development, a visible toner image formed on the photoreceptor drum is transferred to a copy medium by charging the copy medium at a polarity opposite to the toner, using corona discharge of a transfer apparatus. After transfer, the attraction force of the copy medium with the photoreceptor drum is decreased by dropping the electrostatic potential of the copy medium with AC or DC corona discharge, which is then peeled from the photoreceptor drum by its own weight.
In a conventional developing apparatus, a rotatable developing sleeve (developer bearing element) is located adjacent to a rotatable photoreceptor drum. Said developing sleeve, formed as a hollow cylinder, is mounted in a housing, wherein opening of the housing is located opposite the surface of said photoreceptor drum. A developing bias voltage of, for instance, -650V DC plus 2700V AC with a frequency of 8000 Hz is applied to said developing sleeve. A stationary magnet is mounted in the hollow interior of said developing sleeve, while a two component developer, composed of toner powder and magnetic particles (carrier), is attracted to the outer surface of said developing sleeve.
A developing apparatus is comprised of:
a housing means to store a two-component developer composed of toner powder and carrier particles, PA1 a developing sleeve (or a developing roll) as a transfer means of the developer, wherein a magnetic roll, serving as a magnetic field generating means, is mounted PA1 a developer feeding roller (hereinafter, termed a feed roller) PA1 a developer layer regulating member to regulate the thickness of the developer layer formed on the surface of the developing sleeve at a predetermined value, and PA1 a developer agitating screw (hereinafter, referred to as an agitating screw). PA1 a developer storing section for storing two-component developer including toner particles and magnetic carrier particles; PA1 a developing sleeve being a rotatable hollow cylinder; PA1 a first magnetic field generating member provided inside the developing sleeve and having plural magnetic poles to attract the magnetic carrier particles on the developing sleeve so that the developing sleeve conveys the two-component developer with its rotation; PA1 a developer layer thickness regulator to regulate the thickness of the two-component developer on the developing sleeve; PA1 a second magnetic field generating member provided so as to face the developing sleeve and for forming attracting magnetic field to attract the carrier particles from the developing sleeve, wherein the second magnetic field generating member has a closer section located close to the developing sleeve and a farther section located far from the developing sleeve, the farther section is located downstream of the closer section with regard to the conveying direction, and the strength of the attracting magnetic field at the farther section is stronger than that at the closer section so that the carrier particles are removed from the developing sleeve while the magnetic carrier particles is conveyed from the closer section to the farther section by the developing sleeve; and PA1 a developer conveying member provided in the vicinity of the farther section of the second magnetic member and to convey the removed carrier particles to the developer storing section.
The toner in said housing means, which is fed from a toner cartridge through a toner supply opening located at a top portion of said housing means, is mixed and agitated with the developer, stored in said housing means, by means of said agitating screw. As a result, toner of uniform density is supplied on the outer surface of said developing sleeve by means of said feed roller.
In the configuration of the developing apparatus mentioned above, since the toner is attracted to the image bearing element at the developing position, it is required that the residual toner and magnetic carrier should be recycled to the developing sleeve after scrubbing the used developer off the developing sleeve and agitating it with toner at a predetermined ratio.
For this purpose, there has been a technique that the developer is scraped off by physically contacting a scraper blade against the developing sleeve.
According to the above technique, since the scraper blade directly contacts the developing sleeve, a material with high abrasion resistance, such as stainless steel (SUS), etc., should be employed for the developing sleeve. An employment of SUS, however, has required relatively high cost, due to its difficult workability, etc.
Another technique is to remove the developer by means of a magnetic member arranged in a state of non-contacting with an aluminum alloy developing sleeve, which is relatively low in cost, set forth in Japanese Tokkai-sho No.60-91373, Tokkai-sho No.62-105174, Tokkai-hei No.6-194962, etc.
In this technique, a magnetic brush is formed in a space between the developing sleeve and the magnetic member, which is inductively magnetized by a magnet mounted stationary in the hollow interior of the developing sleeve. This magnetic brush acts as a blocking means to scrape the developer off the developing sleeve.
Although the above mentioned technique may be more or less effective to scrape the developer off the developing sleeve, some developer invariably passes through the magnetic brush due to the massive pressure of moving developer on the developing sleeve. Therefore, it may be virtually impossible for this technique to thoroughly remove said large amount of developer off the developing sleeve.
In addition, since the induced magnetic force of the magnetic member is no stronger than that of the magnet mounted stationary inside the developing sleeve, no magnetic force oriented to scrubbing the developer off the developing sleeve is generated. Moreover, some quantity of the developer removed from the developing sleeve directly moves around the developing region of the image bearing element. Such developer would recombine with the surface of the developing sleeve without passing the agitating process, resulting in developing defects.
As mentioned above, it is not effective for this purpose only to arrange the magnetic member in a state of non-contact with the developing sleeve. Especially, the conventional techniques are insufficient for a system where development is achieved by forming a thin layer of the developer on the developing sleeve.